.NET is licensed under MIT[0]. There are additional concessions[1] regarding the code that used to be proprietary specifically to make all the Mono bits to be legally sound.

[0] https://github.com/dotnet/runtime/blob/main/LICENSE.TXT

[1] https://github.com/dotnet/runtime/blob/main/PATENTS.TXT (I think there was another document, please post the link if you remember which one)